Little is known concerning the effect of CO<sub>2</sub> on phytoplankton
ecophysiological processes under nutrient and trace element-limited
conditions, because most CO<sub>2</sub> manipulation experiments have been
conducted under elements-replete conditions. To investigate the effects of
CO<sub>2</sub> and iron availability on phytoplankton ecophysiology, we conducted
an experiment in September 2009 using a phytoplankton community in the iron
limited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin
. Carbonate chemistry was controlled by the bubbling of the several levels of
CO<sub>2</sub> concentration (180, 380, 600, and 1000 ppm) controlled air, and two
iron conditions were established, one with and one without the addition of
inorganic iron. We demonstrated that in the iron-limited control conditions,
the specific growth rate and the maximum photochemical quantum efficiency
(<i>F</i><sub>v</sub>/<i>F</i><sub>m</sub>) of photosystem (PS) II decreased with
increasing CO<sub>2</sub> levels, suggesting a further decrease in iron
bioavailability under the high-CO<sub>2</sub> conditions. In addition, biogenic
silica to particulate nitrogen and biogenic silica to particulate organic
carbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively,
with an increase in the CO<sub>2</sub> level in the iron-limited controls. By
contrast, the specific growth rate, <i>F</i><sub>v</sub>/<i>F</i><sub>m</sub> values and
elemental compositions in the iron-added treatments did not change in
response to the CO<sub>2</sub> variations, indicating that the addition of iron
canceled out the effect of the modulation of iron bioavailability due to the
change in carbonate chemistry. Our results suggest that high-CO<sub>2</sub>
conditions can alter the biogeochemical cycling of nutrients through
decreasing iron bioavailability in the iron-limited HNLC regions in the
future.